Conventionally, atomic hydrogen is produced via dissociation of molecular hydrogen by means of plasma discharge, microwave discharge, dielectric inhibited discharge or by thermal activation using a hot tungsten wire. The generation of atomic hydrogen, however, must be performed in close proximity to the optical surface to be cleaned, since atomic hydrogen recombines and reacts with other substances with high probability and therefore cannot be transported over larger distances without significant loss. This often means that the above mentioned discharge or thermal activation techniques require the corresponding devices to be arranged close to the optical surfaces to be cleaned. In particular in EUV irradiation units the appropriate arrangement of a radiation or plasma source or of hot filaments for generation of atomic hydrogen leads to additional complexity of construction. Furthermore, the impact of a localized plasma can cause an accelerated chemical and/or physical degradation of the optical components, for example of the mechanical stability of the optical components.
EP 1 643 310 A1 describes a method for removal of deposited material on an optical element by exposing the optical element to atomic hydrogen. For the generation of the atomic hydrogen by dissociation of molecular hydrogen several measures are proposed, including the use of a hot filament, the use of plasma radiation or the use of a catalyst configured to convert molecular hydrogen into atomic hydrogen. As an example, a Ru protection layer on the surface of a gracing incidence mirror or of a multilayer mirror, wherein Ru is included in a top layer, is proposed as the catalyst to dissociate molecular hydrogen into atomic hydrogen. The document, however, is silent and does not include any information about the parameters and efficiency of dissociation and cleaning when using such a catalyst.